The present valve mechanism for an internal combustion engine makes it possible to reliably perform switching between the low-velocity operation mode and the high-velocity operation mode without delay. The valve mechanism includes: a first rocker arm which is supported by a rocker shaft in an oscillatory manner; a second rocker arm which is supported by the rocker shaft in an oscillatory manner; a cylinder provided for either said first rocker arm or said second rocker arm, said cylinder communicating with an oil passage; a first piston fitted in said cylinder in a slidable manner; an engaging protrusion provided in a protruding condition for the remaining one of said first rocker arm and said second rocker arm, said engaging protrusion being capable of engaging with said first piston; and a second piston which moves said first piston to an engaging position.